We have previously described the identification of the human decidua-associated protein hDP200 as a rheumatoid factor [1], Assuming that rheumatoid factor binds to Fc of immunoglobulins, the existence of high-molecular-weight complexes was studie in decidual extract, pooled uterine fluid from proliferative and early secretory phase, menstrual fluid and amniotic fluid. High-molecular-weight complexes containing hDP200 and other immunoglobulins were found only in menstrual and amniotic fluids and not in decidual extract or uterine fluid. It is possible that complexing of the immunoglobulins by hDP200 serves as one of the mechanisms that ensure suppression of the immune response to fetal antigens.